Fuel injection system

ABSTRACT

In a fuel injection system, wherein fuel in a fuel tank is pressurized by a fuel pump and the pressurized fuel is fed to a fuel flow control apparatus operating to meter the fuel flow according to the Venturi negative pressure occuring at the Venturi portion in an engine suction system, and the fuel is supplied to the engine in proportion to the suction air quantity, a fuel pressure regulator for controlling the fuel pressure is provided between the fuel pump and the fuel flow control apparatus. The fuel pressure regulator is provided with a fuel pressure compensating apparatus for compensating the fuel pressure controlled by the fuel pressure regulator in accordance with the magnitude of an output signal of an oxygen concentration detector which is provided in an engine exhaust system and produces a signal corresponding to the residual oxygen concentration in exhaust gases. Thus, the accuracy of the mixture ratio of air to fuel which is supplied to the engine suction system is improved and the poisonous components contained in the exhaust gases are reduced.

United States Patent [191 Moriya et al.

[ Oct. 14, 1975 FUEL INJECTION SYSTEM [75] lnventors: Hisanori Moriya, Katsuta; Yasunori Mori, Hitachi, both of Japan [73] Assignee: Hitachi, Ltd., Japan [22] Filed: Sept. 6, 1974 [21] Appl. No.: 503,954

[30] Foreign Application Priority Data Sept. 12, 1973 Japan 48-102111 [52] US. Cl. 123/139 AW; 60/276; 123/32 EA; 123/119 E [51] Int. Cl. F02M 39/00 [58] Field of Search... 123/139 AW, 32 EA, 119 E; 60/276 [56] References Cited UNITED STATES PATENTS 3,288,445 11/1966 Mennesson 123/139 AW 3,504,657 4/1970 Eichler et al. 123/32 EA 3,543,739 12/1970 Mennexon 123/139 AW 3,730,159 5/1973 Sallot. 123/32 EA 3,738,341 6/1973 Loos 123/119 E 3,742,924 7/1973 Bachle 123/119 E 3,796,200 3 1974 Knapp, 60/276 3,815,561 6/1974 Seitz 123/32 EA Primary ExaminerWendell E. Burns Assistant Examiner-James Winthrop Cranson Attorney, Agent, or FirmCraig & Antonelli [57] ABSTRACT In a fuel injection system, wherein fuel in a fuel tank is pressurized by a fuel pump and the pressurized fuel is fed to a fuel flow control apparatus operating to meter the fuel flow according to the Venturi negative pressure occuring at the Venturi portion in an engine suction system, and the fuel is supplied to the engine in proportion to the suction air quantity, a fuel pressure regulator for controlling the fuel pressure is provided between the fuel pump and the fuel flow control apparatus. The fuel pressure regulator is provided with a fuel pressure compensating apparatus for compensating the fuel pressure controlled by the fuel pressure regulator in accordance with the magnitude of an output signal of an oxygen concentration detector which is provided in an engine exhaust system and produces a signal corresponding to the residual oxygen concentration in exhaust gases. Thus, the'accuracy of the mixture ratio of air to fuel which is supplied to the engine suction system is improved and the poisonous components contained in the exhaust gases are reduced.

4 Claims, 6 Drawing Figures U.S. Patent 0a. 14, 1975 Sheet 2 of3 3,911,884

FIG. 2

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OPERATIONAL) AMPLIFIER FUEL INJECTION SYSTEM BACKGROUND OF THE INVENTION The present invention relates to a fuel injection system in which the fuel in a fuel tank is pressurized by a fuel pump and the pressurized fuel is fed to a fuel flow control apparatus operating to meter the fuel flow according to the Venturi negative pressure occurring at the Venturi portion in an engine suction system, and the fuel is supplied to the engine in proportion to the suction air quantity.

With recent more strict requirements for the exhaust gas clearance, a problem of the fuel supply accuracy to the engine has been serious. It is said that the use of reduction catalyst is necessary particularly for restriction of NOx (nitrogen oxide). In this case, the tolerable deviation of the A/F ratio (air flow to fuel flow ratio) is restricted to a considerably narrow range.

As a fuel supply means to an engine, enumerated are a carburetor system and a fuel injection system. These systems are now prevailingly used in most engines. In these systems, however, there are a number of problems to be solved. In the carburetor system, the problems are as follows: The first is that of variations in engine performance, and is due to poor accuracy of the diameter of the fuel metering jet, the air bleed hole, or the like; the second is that of a large variation in the A/F ratio. This problem arises from the fact that this system is based on an idea to control the fuel fiow by the negative pressure occurring in the suction system and hence the heat evolved by the after-burner, the reactor, the catalyst converter, etc., which are provided for cleaning the exhaust gases, provides problems in the fuel suction system, such as percolation, evaporation, vapor locks and the like. In the fuel injection system, there are various ways to control the amount of injection fuel of which the ones utilized in practice are the mechanical control method and the electrical control method. These control methods also have a limitation in improvement of the control accuracy of A/F ratio, as in the carburetor system method. Unless higher accuracy in the A/F ratio control is attained, it is impossible to satisfy the requirements of the exhaust gas regulations. In the fuel injection system, there are two methods for the detection of the suction air flow of reference: one is a method to detect the velocity of the air flow and the density of the air; the other is a method to directly measure the air flow. However, neither of these methods have yet attained a satisfactory accuracy in detection and measurement. This fact provides the problem just mentioned in question.

Further, the A/F ratio accuracy of these current apparatuses of the carburetor and the fuel injection systems is in the order of fl% of a target value. For the accuracy thereof to further approach the target value, a considerable improvement is required in such current apparatuses. Such improvement, however, would result in a complicated and expensive apparatus.

SUMMARY OF THE INVENTION Therefore an object of the present invention is to provide a fuel injection system which improves the A/F accuracy of the fuel-air mixture to be supplied to the engine and reduces poisonous components in the exhaust gases.

Another object of the invention is to provide a fuel injection system capable of obtaining a desired A/F ratio with a simple construction.

To achieve these objects, the present invention has a feature in that the fuel in a fuel tank is pressurized by a fuel pump and the pressurized fuel is fed to the fuel flow control apparatus for metering the fuel flow according to the Venturi negative pressure occurring at the Venturi portion in the engine suction system, through a fuel pressure regulator, and that the fuel pressure regulator is provided with a fuel pressure com pensating apparatus to which an output signal of an oxygen concentration detector is applied, the oxygen concentration detector producing a signal with magnitude corresponding to the concentration of the residual oxygen in exhaust gases, so that an optimum control of the fuel pressure of the fuel delivered from the fuel pressure regulator to the fuel flow control apparatus is attained by the value of the output signal of the oxygen concentration detector. The above and other objects, features, and advantages will be apparent from the detailed description taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic view illustrating the structure of an embodiment of the fuel injection system according to the present invention.

FIG. 2 is a schematic block diagram of the control unit used in the fuel injection system according to the present invention.

FIG. 3 is a particular circuit diagram of the block diagram of FIG. 2.

FIG. 4 illustrates a relationship between the output of the oxygen concentration detector and the energization period of time of the solenoid means.

FIG. 5 is a characteristic curve between air-fuel ratio and the output of the oxygen concentration detector.

FIG. 6 is a schematic view illustrating another embodiment of the fuel injection system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 an air cleaner 1 is fixed to an intake pipe 2. A Venturi 3 is formed in a portion of the intake pipe 2 and a throttle valve 4 is disposed at the downstream to the Venturi portion. Such suction system communicates with a combustion chamber 6A formed within a cylinder 6 through an intake valve 5. A piston 7 is disposed within the cylinder 6. An exhaust pipe 8 communicates with the inside of the cylinder 6 through an exhaust valve 9, and is provided with a reduction catalyst converter 10 and an oxidation catalyst converter 11. 12 is a fuel flow control apparatus which utilizes a well known air-valve carburetor and which operates to keep the negative pressure of the Venturi portion 3 substantially constant by changing the cross-sectional area of the Venturi portion 3 through control of the air flow in the intake pipe. The fuel flow control apparatus 12 is composed of a suction piston 14 slidably mounted in a suction chamber 13, a spring 15 energized so as to push the suction piston 14 downward in the drawing, a jet needle 16 fixed at the lower end portion of the suction piston 14, a metering orifice 17 to control the fuel metering area through the up and down movement of the jet needle 16, and an injection nozzle 18 to inject into the intake pipe 2 the amount of the fuel metered by the metering orifice 17 and the jet needle 16. The'su'ction piston 14 is provided with a suction hole 19'through which the suction chamber 13 communicates with the intake pipe 2. The up and down movement of the suction piston 14 is caused by the variation of the negative pressure exerted on the suction chamber 13 through the suction hold 19. The fuel stored in a fuel tank 20 is sucked through a fuel path 21 into a fuel pump 22. The fuel, after being pressurized in the fuel pump 22, is fed through a fuel filter 23 to a fuel pressure regulator 24 provided with a fuel pressure compensating apparatus. The fuel is regulated therein to the injection pressure approximate to a given value and then is fed to the fuel flow control apparatus 12 through a pressurized fuel path 25. The fuel pressure regulator 24 provided with the fuel pressure compensating apparatus is composed of a fuel pressure control diaphragm 29 energized to normally close a port 28 of a fuel return path 27, the port opening into a fuel control chamber 26, and a valve 30 fixed to the fuel pressure control diaphragm 29. In operation, when the pressure of the fuel fed from the fuel pump 22 is increased to more than a given pressure, the valve 30 operates to open the port 28 thereby permitting communication between the fuel return path 27 and the fuel pressure control chamber 26. This communication therebetween permits the pressurized fuel in the fuel control chamber 26 to flow into the fuel tank 20 through a fuel release path 27A. Thus, the pressure of the fuel is controlled. The fuel arriving at the fuel flow control apparatus 12 through the pressurized fuel path 25, is injected from the injection nozzle 18 after being metered by the metering orifice 17 and the jet needle 16 fixed to the suction piston 14 whose displacement corresponds to the opening of the throttle valve 4. It is to be noted that the injection nozzle 18 opens upstream of the throttle valve 4 and thus it is hardly affected by the pressure of the injecting portion, even if the injection pressure is low. It is to be noted, further, that since that injection nozzle 18 opens in the direction of the sucked air flow no liquid drops are produced by the fuel being brought into contact with the wall of the intake pipe 2.

A description will next be made of the fuel pressure compensating apparatus installed in the fuel pressure regulator. A fuel pressure compensating diaphragm 31 is larger in area than the fuel pressure control diaphragm 29. Those diaphragms 31 and 29 define a hermetically closed fuel pressure compensating chamber 32 which communicates with a fuel pressure switching chamber 34 through a fuel pressure compensating path 33. The fuel pressure switching chamber 34 has a passage through the port 36 to a fuel tank communication path communicating with the fuel tank 20 (the path 35 and the fuel release path 27A are used in common in the drawing),'and another passage through the port 38 to a fuel pressure path 37 communicating with the fuel pump 22 through the outlet thereof. A solenoid 39 serves to drive a rod 40, a directional control valve 41 is fixed to one end of the rod 40, the valve 41 operating to allow the port 36 or the port 38 to communicate with or to be isolated from the fuel pressure switching chamber 34. A rod 42 serves to connect the fuel pressure compensating diaphragm 31 to the fuel pressure control diaphragm 29. An atmospheric pressure chamber 43 is provided with a'spring 44 which is energized so as to normally close the port 28 of the fuel return path 27 with valve 30. An oxygen concentration detector (hereinafter, referred to as an O -sensor) 45 is fitted to the exhaust pipe 8 for detecting the concentration of a residual oxygen contained in the exhaust gases. A control unit 46 receiving the output of the O -sensor through a lead wire 47 produces on-off instructions in response to the deviation of the output of the O -sensor 45 from a reference voltage of a predetermined value, and supplies the instructions through a lead wire 48 to the solenoid 39 thereby to drive the directional control valve 41. The output E of the O -sensor 45 is, given by the following equation:

E KTlog P /P (l) where K is a constant, T is the temperature of the exhaust gas, P is the partial pressure of O in an atmosphere, and P is the partial pressure of O in the exhaust gas.

As seen from the equation (1), the output of the 0,,- sensor 45 is a function of the temperature of the exhaust gas T and the partial pressure P of the 0 contained in the exhaust gas.

The operation of the thus constructed fuel injection system will now be described. The fuel metered by the fuel flow control apparatus 12 is supplied through the intake pipe 2 into the combustion chamber 6A and is burned therein. The burned fuel is exhausted as an exhaust gas through the exhaust pipe 8 to the outside. The O -sensor 45 provided in the exhaust pipe 8 for detecting the concentration of the residual oxygen in the exhaust gas, and produces an output signal corresponding to the detected concentration. More specifically, the output voltage of the O -sensor 45 increases as the airfuel ratio becomes large, as shown in H0. 5. The output signal or the O -sensor 45 is fed to the control unit 46 and is amplified therein and then is applied to the solenoid 39, the directional control valve 41 operates in an ON-OFF mode to switch the passage from the port 36 to the fuel pressure compensating path 33 to the other passage from the port 38 to the fuel pressure compensating path 33, and vice versa. More specifically, when the output voltage of the O -sensor 45 exceeds a certain value representing the desired air-fuel ratio, i.e. when the fuel-air mixture is diluted, the solenoid 39 drives the directional control valve 41 to close the port 38 while opening the port 36 thereby to form the passage from the port 36 to the fuel pressure compensating path 33 for sufficient time for the pressure in the fuel pressure compensating chamber 32 to approach the atmospheric pressure. For this, the port 28 of the fuel return pipe 27 is closed by the valve 30 being pushed up by the spring 44 with the result that the pressure in the fuel pressure control chamber 26 is increased and thus the pressurized fuel is fed through the pressurized fuel path 25 to the flow control apparatus 12 by which is performed a compensating operation for concentrating the fuel-air mixture. On the other hand, when air-fuel ratio is low and thus fuel-air mixture is greater in concentration than the given air-fuel ratio, the output voltage of the O -sensor 45 does not reach the given value. As a result, the solenoid 39 actuates the directional control valve 41 to close the port 36 while opening the port 38 thereby to form the passage between the port 38 and the fuel pressure compensating path 33 for sufficient time for the pressure in the fuel pressure compensating chamber 32 to approach the fuel pressure delivered from the fuel pump 22. At this time, the spring 44 is compressed by the fuel pressure so that the valve 30 which has been keeping the port 28 of the fuel return path 27 closed so far is opened to return the fuel to the fuel tank 20 through the fuel return path 27. For this, the pressure in the fuel pressure control chamber 26 is reduced also to decrease the injection quantity. When the air-fuel ratio is maintained at a predetermined value, the period of time during which the directional control valve 41 operates to open the communicating passage between the port 36 and the fuel pressure compensating path 33 and close the communicating passage between the port 38 and the fuel pressure compensating path 33 is related to the period of time during which the valve 41 operates vice versa at a predetermined ratio. Thus obtained pressure in such a condition as described above is introduced into the fuel pressure compensating chamber 32 and balances with a force which depends on the difference between the areas of the two diaphragms 29 and 31, and the force of the spring 44 balances with the pressure in the fuel pressure control chamber 26, thus obtaining a reference fuel pressure.

The control unit 46 will next be described in detail with reference to FIGS. 2, 3, 4 and 5. In FIG. 2, the output signal of the O -sensor 45 is amplified by an amplifier 49 to a sufficiently high voltage. The output of an unstable multivibrator 50 is converted at an integration circuit 51 into an approximately triangular wave form. The triangular-wave form is superposed on a reference DC component. A comparator 52 operates to compare the amplified output of the O -sensor 45 with the integrated output of the unstable multivibrator 50. The output of the comparator 52 is amplified in an amplifier 53 and fed to the solenoid 39 to mechanically drive the directional control valve 41. p

The actual circuit diagram of the schematic block diagram in FIG. 2 will now be described with reference to FIG. 3. Like reference numerals are used therein to designate like portions in FIG. 2. The output of the O sensor 45 is amplified by the amplifier 49 composed of resistors R R R R and R and an operational amplifier OP to the voltage of V shown in FIG. 4. The amplifier output voltage V is applied to the base of a transistor of the comparator 52 which is a differential amplifier. The unstable multivibrator 50 is comprised of transistors Q and Q capacitors C and C resistors R R R and R and its output is supplied to the integration circuit 51 consisting of a resistor R and capacitor C The output of the unstable multivibrator 50 charges the capacitor C when the transistor O is not conductive and the charged capacitor C discharges when the transistor O is conductive. As a result, an approximate triangular wave form V is obtained as shown in FIG. 4. The triangular wave form V, is applied to the base of a transistor 0:, which is a constituent of the comparator 52. The comparator 52 is comprised of resistors R to R and transistors 0 to Q The triangular wave produced by the unstable multivibrator 50 and the integration circuit 51 is constant. When the output of the O -sensor 45 is increased, i.e. the fuel-air mixture is diluted, the output of the 0,- sensor 45 is also increased as shown in FIG. 5 and is a base'voltage of the transistor 0 At this time, the transistor O is turned on during only the period of time t corresponding to the portion where the triangular wave V, is higher than the amplified output voltage V of the O -sensor 45, as shown in FIG. 4. When the transistor 0;, is turned on, transistors Q Q1 and Q of the amplifier 53 are also turned on so as to amplify this signal to the voltage V as shown in FIG. 4, so that a current flows through the solenoid 39. The amplifier 53 is comprised of resistors R, to R and the transistors O to Q and a diode D. With the current flowing through the solenoid 39 for the period of time t, the directional control valve 41 shown in FIG. 1 closes the port 36 for the time t and opens the same for the period of time (T t) which is the difference between the period T of the multivibrator 50 and the period of time t of the conduction of transistors mentioned above. Onthe contrary, the port 38 is opened for the time t while it is closed for the time (T I). That is, the amplified output voltage V of the O -sensor determines the ratio r/T. It should be noted that if the period T is selected to be considerably smaller than the response time of the directional control valve 41 which is driven by the solenoid 39, it is possible to control the stroke of the directional control valve 41 actuated by the solenoid 39 depending on the ratio t/T. Thus, when the fuel-air mixture fed to the engine is diluted, the concentration of the residual oxygen in the exhaust gas is high so that the output voltage of the Q -sensor 45 becomes higher and thus the period of time of the energization of the solenoid 39 is shortened. As a result, the port 36 is opened for a long time and the pressure of the fuel pressure compensating chamber 32 approaches the atmospheric pressure and the valve 30 operates to close the port 28 of the fuel return path 27 and makes the pressure in the fuel pressure control chamber 26 high, so that the flow of the fuel supplied to the engine is increased. In the case of high concentration of the fuel-air mixture, the output V of the O -sensor 45 decreases and the conduction time of the transistor O is increased with an elongation of the conduction time of the current flowing through the solenoid 39. For this, the directional control valve 41 closes the port 36 for a long time while at the same time opens the port 38 for a long time. Accordingly, the fuel pressure of the fuel pump 22 is exerted on the fuel pressure compensating chamber 32 and the valve 30 opens the port 28 of the return path 27 to permit the fuel in the fuel pressure control chamber 26 to return to the fuel tank 20, so that the fuel pressure is reduced and therefore the fuel flow to the engine is also reduced.

As described above, the present invention is constructed such that the oxygen concentration in the exhaust gas is detected and the fuel pressure of the fuel to be supplied to the fuel flow control apparatus is compensated by the detected signal. As a result, accuracy of the air-fuel ratio is improved andd further the manufacturing cost may be reduced. In the embodiment described above, the fuel pressure control diaphragm of the fuel pressure regulator is smaller in area than the fuel pressure commpensating diaphragm so that the fuel pressure in the fuel pressure control chamber is controlled to be kept low with a high fuel pressure in the fuel pressure compensating chamber. Inversely, however, the area of the fuel pressure control diaphragm may be made larger than that of the compensating chamber so that the pressure in the fuel pressure control chamber may be made high by increasing the pressure in the fuel pressure compensating chamber. In this case, the pressure of the reference fuel must be selected to be low.

Additionally, in the embodiment described above, for the fuel flow control apparatus the well known airvalve type carbureter is utilized which operates to keep the Venturi negative pressure constant. However, it is possible to employ another type of fuel flow control apparatus as shown in FIG. 6. In the figure, like reference numerals are used to designate like or equivalent parts of portions to those shown in FIG. 1. 3A is Venturi portion with a constant cross sectional area. A nozzle tube 54 with an injection nozzle 18A projects into the Venturi portion 3A and, more particularly the nozzle opens at the approximate center of the Venturi portion 3A. The Venturi negative pressure is introduced into a negative pressure chamber 55 of the fuel flow control apparatus, through a negative pressure passage 58 opening into Venturi portion 3A, and pressure amplifier 57. The fuel flow control apparatus 12A is comprised of a spring 56, a diaphragm 59, and a metering needle 16A fixed to the diaphragm 59. The negative pressure occuring at the Venturi portion 3A is introduced into the pressure amplifier 57 through the negative pressure passage 58 and amplified therein and then applied to the diaphragm 59 forming the negative pressure chamber 55. On application of the negative pressure to the diaphragm 59, the metering needle 16A meters the fuel in cooperation with a metering orifice 17A in response to the amount of the applied negative pressure. Then, the metered fuel is injected from the injecting nozzle. As in the embodiment of FIG. 1, the fuel whose pressure is regulated by the output of the O -sensor 45, is supplied to the fuel flow control apparatus 12A.

It should also be noted that the present invention may be applicable to a mechanical fuel injection apparatus and an electrical fuel injection apparatus besides a fuel flow control apparatus such as described above.

We claim:

1. A fuel injection system comprising:

a fuel pump means for pressurizing fuel in a fuel tank;

fuel pressure regulator means for controlling the fuel pressure of the fuel pressurized by said fuel pump to keep the fuel substantially in a predetermined value;

fuel compensating means, provided in said fuel pressure regulator means, for compensating the fuel pressure to be controlled by said fuel pressure regulator means in response to the magnitude of an output signal of oxygen concentration detector means which produces a signal with a value corresponding to residual oxygen concentration in exhaust gases in an engine exhaust system; and

fuel flow control means for metering and injecting the pressure compensated fuel in acccordance with the Venturi negative pressure occurring at a Venturi portion provided in an engine suction system.

2. A fuel injection system according to claim 1; wherein said fuel pressure regulator means comprises a fuel pressure control chamber; first, second and third passages opening into said fuel pressure control chamber for communicating with said fuel pump means, with said fuel flow control means, and with said fuel tank respectively; and first valve means, fixed on a first diaphragm, for operating to alternatively close and open said third passage in response to the pressure in said fuel pressure control chamber; and wherein said fuel pressure compensating means comprises a fuel pressure compensating chamber defined by said first diaphragm and a second diaphragm; fourth and fifth passages opening into said fuel pressure compensating chamber for communicating said fuel pressure compensating chamber with said fuel pump and with said fuel tank respectively; second valve means for switching said fourth and fifth passages in a manner so that said fuel pressure compensating chamber alternatively communicates with said fuel pump and said fuel tank means for driving said switch valve means in response to an output signal of said oxygen concentration detector means.

3. A fuel injection system according to claim 2, wherein means for energizing said solenoid means is provided to cause said solenoid means to drive said second switch valve means, said solenoid energizing means including oscillating means with an oscillating frequency whose period is shorter than the response time of said second switch valve means; means for converting the output of said oscillating means into an approximately triangular wave; and means for comparing said approximatley triangular wave with the output of said oxygen concentration detector means, whereby the ratio of energizing period to de-energizing period of said solenoid is controlled by the output of said comparing means.

4. A fuel injection system according to claim 3, wherein said solenoid energizing means includes an unstable multivibrator with a frequency whose period is shorter than the response time of said solenoid an integration circuit for converting the output of said unstable multivibrator into a triangular wave; a first amplifier for amplifying the output of said oxygen concentration detector; a differential amplifier for comparing the output of said integration circuit with the output of said first amplifier; and a second amplifier for amplifying the output of said differential amplifier, the output of said second amplifier being applied to said solenoid means. 

1. A fuel injection system comprising: a fuel pump means for pressurizing fuel in a fuel tank; fuel pressure regulator means for controlling the fuel pressure of the fuel pressurized by said fuel pump to keep the fuel substantially in a predetermined value; fuel compensating means, provided in said fuel pressure regulator means, for compensating the fuel pressure to be controlled by said fuel pressure regulator means in response to the magnitude of an output signal of oxygen concentration detector means which produces a signal with a value corresponding to residual oxygen concentration in exhaust gases in an engine exhaust system; and fuel flow control means for metering and injeccting the pressure compensated fuel in acccordance with the Venturi negative pressure occurring at a Venturi portion provided in an engine suction system.
 2. A fuel injection system according to claim 1; wherein said fuel pressure regulator means comprises a fuel pressure control chamber; first, second and third passages opening into said fuel pressure control chamber for communicating with said fuel pump means, with said fuel flow control means, and with said fuel tank respectively; and first valve means, fixed on a first diaphragm, for operating to alternatively close and open said third passage in response to the pressure in said fuel pressure control chamber; and wherein said fuel pressure compensating means comprises a fuel pressure compensating chamber defined by said first diaphragm and a second diaphragm; fourth and fifth passages opening into said fuel pressure compensating chamber for communicating said fuel pressure compensating chamber with said fuel pump and with said fuel tank respectively; second valve means for switching said fourth and fifth passages in a manner so that said fuel pressure compensating chamber alternatively communicates with said fuel pump and said fuel tank means for driving said switch valve means in response to an output signal of said oxygen concentration detector means.
 3. A fuel injection system according to claim 2, wherein means for energizing said solenoid means is provided to cause said solenoid means to drive said second switch valve means, said solenoid energizing means including oscillating means with an oscillating frequency whose period is shorter than the response time of said second switch valve means; means for converting the output of said oscillating means into an approximately triangular wave; and means for comparing said approximatley triangular wave with the output of said oxygen concentration detector means, whereby the ratio of energizing period to de-energizing period of said solenoid is controlled by the output of said comparing means.
 4. A Fuel injection system according to claim 3, wherein said solenoid energizing means includes an unstable multivibrator with a frequency whose period is shorter than the response time of said solenoid an integration circuit for converting the output of said unstable multivibrator into a triangular wave; a first amplifier for amplifying the output of said oxygen concentration detector; a differential amplifier for comparing the output of said integration circuit with the output of said first amplifier; and a second amplifier for amplifying the output of said differential amplifier, the output of said second amplifier being applied to said solenoid means. 